In all the discussion about how to “lean in,” one aspect gets routinely overlooked: Childless women often don’t have a choice.

At their offices and workspaces, the demand from parents for time off means single women without kids are routinely pressured into working late, scheduling vacations for off-seasons, and otherwise picking up the slack that work/life balance leaves undone by their colleagues. Ayana Bird at Marie Claire reports:

It's the newest form of workplace discrimination: single, childless women who carry an undue burden at the office, batting cleanup for their married-with-kids coworkers.

Facebook Chief Operating Officer Sheryl Sandberg's best-selling book, Lean In: Women, Work, and the Will to Lead, makes a strong case for women fully committing to their careers, but this kind of non-optional "leaning in" is not what she's advocating. Instead, it's an inequity simmering under the surface in many corporate cultures, says social scientist Bella DePaulo, Ph.D., author of Singlism: What It Is, Why It Matters, and How to Stop It.

According to DePaulo, "singlism" represents the myriad ways that our culture rewards married couples, from discounts on car insurance to preferential treatment in the housing market, while treating singles as second-class citizens—and it's increasing in the office.

"When almost half of the people in the U.S. are single, why do companies continue to cater to their employees who are married with children?" asks DePaulo.

Bird recounts examples that should be familiar to anyone who’s worked in an office before: Married mothers getting vacation time to help create Halloween costumes for their kids while single, childless women can’t even get time off to visit their parents.

Women missing dates, exercise classes, and social outings in order to cover for the mothers they work with. (What about the men? As some fathers take on more responsibility at home, they too are creating extra work for their child-free colleagues. And single men, who are often ignored in the "lean in" debates, can end up picking up the slack, too.) Ann Friedman complained at the Cut in March:

Many corporations now strive for a veneer of family friendliness, so it’s not likely a woman will get the stink-eye for leaving early to catch her kid’s soccer game. Which is a feminist victory.

But if a childless employee cops to the fact that she’s ducking out for a yoga class? It’s seen as downright indulgent and may even show up on a performance review.

One reason it’s become acceptable to squeeze childless women for more work while indulging mothers is that it’s assumed that what goes around comes around.

Women who are currently staying late so a co-worker can go to Timmy’s baseball will eventually have their own kids and lean on the next gen of childless singles.

But as increasing numbers of women opt out of the marriage-and-babies game altogether, that assumption can’t be counted on any longer.

Of course, the flip side is that for some women, all this ability to work harder and longer means that you get to be taken more seriously as an employee and get looked at more for raises and promotions.

But like many things, it depends on the situation you’re in. As Mary Ann Mason explained recently here at Slate, being a single, childless woman in the world of academia means getting taken more seriously as a professional and having a chance at more prestigious jobs.

Outside the world of elite professionals, however, it’s a different story.

Research shows that when it comes to more ordinary working class and administrative jobs, single, childless women may be beloved on the job for their ability to work long hours, but their rejection of traditional gender roles means their colleagues view them with suspicion and subject them to more harassment and abuse.

Clearly, what we need are work environments where the specifics of your home life shouldn’t matter at all, and a culture that supports everyone having a life outside of work, regardless of whether that life includes raising kids or not.